When humans blacktop a certain stretch of land, it takes away the natural drains and sponges created by the vegetation. The water builds up and since there is no where to it to go, it sits in large puddles on the tarmac. A good example of this was when the city of North Vancouver built the Westview overpass. With all the new tarmac and concrete in place of the grassy median, the water had no where to infiltrate the soil, or to runoff. So massive puddles formed at the low point of the overpass, and stayed there until road crews installed a proper drainage system. Also with the new drainage system there were pipes that broke and caused the liquefaction of some of the upper portion of the new highway. Causing an unevenness of the tarmac and the total reconstruction of the highway